RFR: 8252935: Add treeShowing listener only when needed [v2]

[John Hendrikx]

On Fri, 22 Jan 2021 21:36:53 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> John Hendrikx has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains one commit:
>> 
>>   WIP: Moved treeShowingProperty into its own class
>
> I left a few inline comments below.

Sorry about the force push, merging over a year of changes from master did not seem right to me.  It was only for getting up to date, I will do the other commits normally.

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PR: https://git.openjdk.java.net/jfx/pull/185